Aging is associated with a reduced capacity to cope with physiological stress. To study the molecular mechanisms associated with the decline in stress tolerance that accompanies aging, differences in gene expression between young and old Fischer 344 rats under euthermic control conditions or in response to hyperthermic challenge were evaluated using a cDNA array containing 207 stress-related genes. In the nonstressed control condition, aging resulted in selective upregulation of stress protein genes and transcripts involved in cell growth, death, and signaling, along with a downregulation of genes involved in antioxidant defenses and drug metabolism. Heat stress resulted in a broad induction of genes in the antioxidant and drug metabolism categories and transcripts involved in DNA, RNA, and protein synthesis for both age groups. Old animals had a robust upregulation of genes involved in cell growth, death, and signaling after heat challenge, along with a blunted expression of stress-response genes. In contrast, young animals had a strong induction of stress-response genes after hyperthermic challenge. Changes in expression of selected genes were confirmed by RT-PCR analysis. These findings suggest that aging results in altered gene expression in response to heat stress that is indicative of decreased stress protein transcription and increased expression of oxidative stress-related genes. Thus our findings support the postulate that transcriptional changes in response to a physiological challenge such as hyperthermia contribute to the loss of stress tolerance in older organisms.